1. Field of the Invention
The present invention relates to a route search apparatus that may be adopted in navigation systems for vehicles and the like to search a route from a point of departure to a destination.
2. Description of the Related Art
Vehicular navigation systems in the known art are provided with a function for displaying a roadmap of the area where the vehicle is currently located, a function for accurately detecting the vehicle position through map matching, a function for calculating a recommended route from a point of departure to a destination and the like. In these vehicular navigation systems in the prior art, roadmap display data, data for map matching and data for route search are separately stored in a CD ROM.
The roadmap display data include widest range map data for displaying large areas at the smallest scale, most detailed map data for displaying small areas in detail at the largest scale and a plurality of sets of map data at different scales between that of the widest range map data and that of the most detailed map data. For instance, the widest range map data may be referred to as level 4 data, the most detailed map data may be referred to as level 1 data and the sets of data at scales between those of the level 4 data and the level 1 data may be referred to as level 3 data and level 2 data. And the route search data include two sets of data that correspond to the level 4 and level 2 roadmap data, and the vicinity of the point of departure and the vicinity of the destination are searched using the level 2 data and the other area is searched using the level 4 data for route search to reduce the length of time required for the route search.
However, when one or more waypoints are set between a point of departure and a destination, read out of level 2 route search data and read out of level 4 route search data are mixed, and even if a buffer memory or the like is provided between the CD ROM device and the control device, the read out data are not hit with a high degree of efficiency in the buffer memory, resulting in a reduction in processing speed.
Moreover, it is desirable that the processing time required for route search be reduced, since, once the user issues an instruction for route search start to the system after setting a destination and the like, the results of the route search should be output as soon as possible so that the user will not have to wait for the results.
Also, when calculating a recommended route from a point of departure (current vehicle position) to a destination, a start point is set for route search in the vicinity of the point of departure using the roadmap display data, an end point is set for route search in the vicinity of the destination in a similar manner and then the route search data are used to perform route search between the start point and the end point that have been set. In this process, the closest road (the minimum unit of a road is referred to as a link) is selected as the start point or the end point of the route search.
However, if such a road (link) is under a traffic restriction such as one-way traffic and the vehicle cannot advance toward the destination from the point of departure or if the vehicle cannot approach the destination from the direction of the point of departure, the route search is abandoned and results are output indicating that the route search could not be completed.